Composite materials, for use in the aerospace industry, are well-known to the art. Such materials comprise a resinous binder, as for example a polymerized epoxide and a filler, as for example asbestos, glass fibers, or carbon fibers.
Of the above named fillers, carbon fibers have received attention due to their high corrosion and temperature resistance, low density, high tensile strength and high modulus of elasticity.
Uses for such carbon-fiber reinforced composites include aerospace structural components, rocket motor casings, deep submergence vehicles, and ablative materials for heat shields on re-entry vehicles.
The incorporation of carbon or graphite particles in resin bases in amounts of up to 60 percent by volume will impart a heat-conducting property but not an electrical conductivity to the component. However, Litant, in U.S. Pat. No. 3,406,126, teaches that the addition of a carbon yarn in as little as 0.05 percent by volume to the resinous matrix imparts electrical conductivity to the resulting composite. Such composites can be prepared from polyesters, polyvinyl chloride, polyepoxides, or like resins, and carbonized rayon, polyacrylonitrile, or like fibers.
Composites containing Acrilan, Orlon, polyacrylonitrile, rayon, and like-based carbon fibers have been described by Litant. These composites are electrically conductive materials which are useful as electrical brushes and contacts, and as structural units in conductive flooring, wall panelling, and the like.
When composites such as these are manufactured from a highly oriented precursor as for example from carbon fibers, stretched and graphitized by the method of Prescott U.S. Pat. No. 3,533,743 or Spry U.S. Pat. No. 3,454,362, a high modulus composite is produced.
Such high modulus composites usually have low interlaminar shear strengths of about 3000 to 4000 p.s.i. These low shear strengths are probably due to poor bonding between the carbon fibers and the matrix. Attempts to improve this bonding, particularly between rayon-based carbon fiber fillers and an epoxy-matrix have been partially successful, but have resulted in a degradation of the ultimate tensile strength of the fiber and also of the fabricated composite.
Improved bonding has been accomplished by plating the fiber with various metals, for example tantalum, with metal carbides, as for example whiskers of silicon carbide, and with nitrides.
More recently, rayon-based carbon fibers have been treated with various oxidizing agents in order to etch the surface of the fiber. Such oxidizing agents have included air, ozone, concentrated nitric acid, and a 3.3 percent by weight solution of sodium dichromate in concentrated sulfuric acid at 50.degree. C. for 5 minutes. In most cases the oxidative treatment of rayon-based carbon fibers resulted in a decrease in ultimate tensile strength of the fiber and of the fiber-resin composite.
The primary structural properties of composites improve as carbon fiber content is increased up to about 65 volume percent then decrease as the fiber content exceeds that aforementioned figure. The preferred range of carbon fiber content is about 45 to 65 volume percent of fiber in the fabricated composite.